1. Field of the Invention
The present invention relates to the purification of alkylene oxides such as propylene oxide by contact with solid activated carbon adsorbent and especially to a process wherein the activated carbon is first contacted and prewetted with a glycol such as propylene glycol before contact with the alkylene oxide whereby excessive exotherms caused by the heat of adsorption are substantially avoided; in addition, a substantially improved carbon bed changeover procedure is provided.
2. Description of the Prior Art
Methods are known in the art for the purification of alkylene oxides such as propylene oxide by contact with solid activated carbon.
U.S. Pat. No. 4,692,535, for example, shows separation of high molecular weight poly (propylene oxide) polymer from propylene oxide by contact with activated carbon.
U.S. Pat. No. 5,187,287 and related U.S. Pat. No. 5,352,807 show the separation of various organic impurities from lower alkylene oxides such as propylene oxide and butylene oxide by activated carbon treatment.
There are various difficulties associated with the treatment of propylene oxide with activated carbon. Due to the considerable heat released upon adsorption of propylene oxide on activated carbon there are both hazards and possible damage to the carbon bed associated with excessive temperature increases during the initial or start-up phase of the activated carbon treatment process. Where the bed is contacted with liquid propylene oxide, the accompanying adsorption exotherm has resulted in propylene oxide vaporization and migration in the bed which in turn causes secondary exotherms with temperatures in excess of 500.degree. C. with extreme hazard and reactor damage. During bed changeover, a major concern has been the handling of toxic and hazardous carbon/propylene oxide/water slurries which pose safety and environmental hazards.
Prior carbon treatment procedures have required vapor recovery systems, the provision of steam, nitrogen, cooling, flare facilities as well as sophisticated control systems and trained operators.
Difficulties are compounded by the fact that high molecular weight poly (propylene oxide) polymer can be formed during shipping and storage of propylene oxide and this polymer formation has a pronounced adverse effect on the use of polyols formed from the propylene oxide in polyurethane foams. U.S. Pat. No. 4,692,535 provides a detailed description of this. Although propylene oxide can be purified at the source, impurities form during shipping and storage, and frequently it has not been practical to provide the sophisticated facilities and operation at various locations where the propylene oxide is used to accomplish activated carbon treatment by prior procedures.